Re: [PATCH v3 01/13] mm/slab: skip kfence objects in allocation profiling

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 7/30/26 13:16, Hao Li wrote:
> On Mon, Jul 27, 2026 at 02:53:55PM +0200, Vlastimil Babka (SUSE) wrote:
>> struct kfence_metadata only contains struct slabobj_ext with
>> CONFIG_MEMCG, which is then used for the "fake" slab's obj_exts field.
>> If CONFIG_MEMCG is enabled, the struct can also end up used for memory
>> allocation profiling. If CONFIG_MEMCG is disabled but profiling is
>> enabled, it will end up allocating its obj_exts via
>> prepare_slab_obj_exts_hook() and assigning them to the fake struct slab.
>> These will probably then never be freed.
>> 
>> So things sorta work, but not always in the intended and optimal way.
>> The upcoming changes to slabobj_ext layout would additionally need a
>> proper refactoring to keep working.
>> 
>> However, there's little benefit in accounting KFENCE objects. KFENCE
>> allocations are rare and there can be only CONFIG_KFENCE_NUM_OBJECTS
>> (default to 255) outstanding ones at any time. For any callsite
>> prominent enough in the memory allocation profiling stats, allocations
>> served from KFENCE will be lost in the noise.
>> 
>> Thus let's not complicate things and simply stop accounting KFENCE
>> objects in allocation profiling and skip them in the related slab hooks.
>> 
>> We also need to skip kfence objects in mark_obj_codetag_empty() in case
>> a sheaf is allocated from kfence, per earlier sashiko review.
>> 
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>> ---
>>  Documentation/mm/allocation-profiling.rst | 7 +++++++
>>  mm/slub.c                                 | 9 +++++++++
>>  2 files changed, 16 insertions(+)
>> 
>> diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst
>> index 5389d241176a..d02eb54ee8f2 100644
>> --- a/Documentation/mm/allocation-profiling.rst
>> +++ b/Documentation/mm/allocation-profiling.rst
>> @@ -112,3 +112,10 @@ break it out by rhashtable type.
>>  
>>  - Then, use the following form for your allocations:
>>    alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...))
>> +
>> +Notes
>> +=====
>> +
>> +- When a slab object is allocated from KFENCE, its accounting is skipped.
> 
> should this be 'allocation profiling is skipped' instead?

The document uses the word "accounting" when it comes to individual
allocations, so this fits.

> Otherwise, looks good to me.
> Reviewed-by: Hao Li <hao.li@linux.dev>

Thanks!